Issue 52

A. Laureys et alii, Frattura ed Integrità Strutturale, 52 (2020) 113-127; DOI: 10.3221/IGF-ESIS.52.10 115 Wt% S C N P Mn Ti Al Si Fe ULC steel 38 ppm 214 ppm 88 ppm 73 ppm 0.25 0.002 0.047 Balance TRIP steel 0.17 0.04-0.1 1.60 1-2 0.40 Balance Fe-C-Ti steel 0.1 50 ppm 0.38 0.03 Balance Table 1: Chemical composition of ULC steel, TRIP-assisted steel and Fe-C-Ti steel. Wt% C Si Mn P S Cr ≤ 0.25 0.10-0.30 1.15-1.55 ≤ 0.015 ≤ 0.012 ≤ 0.25 Wt% Cu Mo Ni V Al Fe ≤ 0.20 0.45-0.55 0.50-0.80 ≤ 0.03 ≤ 0.04 Balance Table 2: Chemical composition range specifications for the two pressure vessel alloys (wt%). Ultra-low carbon steel was used as a material of study in order to avoid the effect of complex microstructural characteristics on blister/internal crack characterization. A cold deformed ULC steel is studied in the current study, since controllable introduction of blisters is possible in this material [6]. The corresponding microstructure is shown in Fig. 1. This material consists of deformed ferrite grains and a small number of Al 2 O 3 inclusions. Only a few hydrogen trapping sites are present in this material, i.e. grain boundaries, dislocations, vacancies, microvoids and additionally, some inclusions. Figure 1: Dark field optical microscopy image of cold deformed ULC steel. Reprinted with permission from Ref. [6]. Figure 2: SEM image of a) TRIP assisted steel (RA: retained austenite, M: martensite, F: ferrite, B: bainite) and b) original AlN inclusions/voids in the steel. Reprinted with permission from Ref. [40]. Contrary to ULC steel, TRIP-assisted steel has a complex multiphase microstructure with a ferritic matrix and a dispersion of multiphase grains of bainite, retained austenite and some martensiet [41] (Fig. 2). The ferritic grains appear as big, flat